ASYNCIFY=1 does not work with functions that need to return a value back to JS

ASYNCIFY=1 does not work with functions that need to return a value back to JS side, for example `emscripten_request_animation_frame_loop()`.

`a.cpp`

```c++
#include
#include

extern "C" void wgpu_buffer_map_sync(int buffer, int mode, double offset, double size);

EM_BOOL raf(double time, void *userData)
{
EM_ASM(console.log(`rAF at time ${$0}`), time);

// Map a GPU buffer synchronously
wgpu_buffer_map_sync(1,2,3,4);

// Return EM_TRUE to signal we want to keep the rAF loop going on.
return EM_TRUE;
}

int main()
{
emscripten_request_animation_frame_loop(raf, 0);
}
```

`lib_a.js`

```js
mergeInto(LibraryManager.library, {
wgpu_buffer_map_sync__sig: 'viidd',
wgpu_buffer_map_sync__async: true,
wgpu_buffer_map_sync: (i1, i2, d1, d2) => Asyncify.handleAsync(async () => {
await 42;
}),
});
```

```
em++ a.cpp --js-library lib_a.js -sASYNCIFY=1 -o a.html
```
The intent with this program is to have the application render continuously in a rAF loop, and map a WebGPU buffer in each tick of the loop.

However, `wgpu_buffer_map_sync()` ends up returning integer `0` back to caller, and the frame loop will not be set up to continue.

(With brief testing, it seems that maybe `-sASYNCIFY=2` does not have this issue)

I wonder if there is any way to fix the above issue?